1. Field of the Invention
The present invention relates to an image pickup apparatus having a power zoom lens, automatic focusing means, and a power turning camera platform, which is used in, for example, a teleconference system or a remote monitor system.
2. Related Background Art
In a television camera, an image signal obtained by exposing an object image to an image pickup element such as a CCD or CMOS for a given period of time is converted into a digital signal, and then subjected to a given process such as a YC process to obtain an image signal of a given format.
There has been known an automatic focus system in which the sharpness level of a screen is detected according to an image signal of the object image, and a focus lens position is controlled so that the sharpness level becomes maximum to conduct focusing.
As an evaluation of the sharpness level of the image, there is used a signal indicative of the intensity of a high frequency component obtained by allowing the image signal to pass through a band pass filter (BPF), or a signal indicative of the sharpness level such as a blur range of the image signal extracted by differentiation.
In the case where a normal object is photographed, the signal indicative of the sharpness level is small in a defocusing state and becomes larger as the focus is clearer. The signal becomes maximum in the focusing state.
Accordingly, as a control of a focus lens, after the focus lens is driven at a high speed in a direction in which the sharpness level is increased when the sharpness level is small, the sharpness level is inverted when the sharpness level switches over from the increase to decrease, and thereafter the focus lens stops at a mountain summit (an area where the sharpness level becomes maximum).
This system is widely known as a Maximum Power Point Tracking automatic focus control system indicated by a line A of FIG. 9.
However, in the automatic focusing device using the image signal, the dependency on the object is high.
For that reason, as indicated by a line B of FIG. 9, the mountain (change) per se of the sharpness signal is gentle in the object low in the contrast, and therefore it may be very difficult to detect the summit.
Also, because AF cannot be conducted, there may occur a trouble that hunching occurs.
Also, in a camera system used for monitoring, there are various conditions such as scenes in which the contrast is extremely low in the night, or there is no object to be monitored.
In that case, focusing operation becomes inactive.